It will be disabled by default if it detects that STDOUT is not a TTY.

Installation

geminstallrainbow

Usage

require'rainbow'puts"this is red".foreground(:red)+" and "+"this on yellow bg".background(:yellow)+" and "+"even bright underlined!".underline.bright

Rails Usage

You're probably wanting to add colour to your logs. To do so you must explicity enable rainbow because it will detect that STDOUT (ie: the log file) is not a TTY.
To make things easy, create the file config/initializers/rainbow.rb and include the following: